&nbsp326 &nbspFXUS02 KWBC 241600 &nbspPMDEPD &nbsp &nbspEXTENDED FORECAST DISCUSSION &nbspNWS WEATHER PREDICTION CENTER COLLEGE PARK MD &nbsp1059 AM EST SAT FEB 24 2018 &nbsp &nbspVALID 12Z TUE FEB 27 2018 - 12Z SAT MAR 03 2018 &nbsp &nbsp...ANOTHER HEAVY RAIN EVENT FOR PARTS OF THE MID-MISSISSIPPI &nbspVALLEY/MID-SOUTH AND ONGOING FLOOD CONCERNS FROM SATURATED &nbspSOILS/SWELLING RIVERS... &nbsp &nbsp&nbsp &nbsp..PATTERN OVERVIEW &nbsp &nbspEXPECT SOME ADJUSTMENT IN THE NORTH AMERICAN FLOW PATTERN DURING &nbspTHE MEDIUM RANGE PERIOD. A BUILDING UPPER RIDGE ACROSS THE HIGH &nbspLATITUDES OF THE NORTH ATLANTIC WILL PROMOTE DEVELOPMENT OF A REX &nbspBLOCK ALONG APPROXIMATELY 30 DEG W LONGITUDE AT THE START OF THE &nbspFORECAST DAY 3 TUE. THE POSITIVE HEIGHT ANOMALY CENTER WILL &nbspGRADUALLY SHIFT WESTWARD AND LIKELY REACH THE LABRADOR SEA BY NEXT &nbspFRI-SAT, WITH THE AXIS OF THE REX BLOCK DIPOLE SHIFTING WEST AS &nbspWELL. AT THE SAME TIME POSITIVE HEIGHT ANOMALIES WILL ALSO BUILD &nbspNORTH FROM THE EAST-CENTRAL PACIFIC ACROSS ALASKA AND INTO THE &nbspARCTIC OCEAN. THE IMPLICATIONS OF THIS EVOLUTION FOR WEATHER &nbspACROSS THE CONUS ARE SOMEWHAT UNCLEAR. TELECONNECTIONS WITH &nbspRESPECT TO TEMP/PRECIP ANOMALIES ACROSS THE CONUS ARE HIGHLY &nbspSENSITIVE TO THE EXACT POSITION OF THE NORTH ATLANTIC RIDGE. IF &nbspTHIS NEGATIVE NAO/AO PATTERN CAN PERSIST FOR LONG ENOUGH, IT WOULD &nbspFOSTER A COLDER AND DRIER PERIOD FOR THE EASTERN SEABOARD THOUGH &nbspLIKELY BEYOND THE END OF THIS FORECAST PERIOD. THERE IS AT LEAST &nbspLESS SENSITIVITY TOWARD THE IDEA OF THE CORE OF NEGATIVE HEIGHT &nbspANOMALIES REMAINING WELL OFF THE EAST COAST TOWARD 60W LONGITUDE. &nbspOUT WEST, THE IMPLICATIONS SEEM CLEARER FOR A TIME WITH &nbspSTRENGTHENING RIDGING ACROSS THE NORTH PACIFIC/ALASKA SUPPORTING &nbspPERSISTENT TROUGHING FROM THE WEST COAST TO THE NORTHERN ROCKIES. &nbspHOWEVER A GRADUAL WESTWARD SHIFT IN EMPHASIS OF STRONGEST POSITIVE &nbspHEIGHT ANOMALIES OVER THE NORTH PACIFIC COULD LEAD TO SOME &nbspFLATTENING OF EASTERN PACIFIC/WESTERN U.S. FLOW (IN MULTI-DAY MEAN &nbspTERMS) AROUND OR AFTER THE END OF THE PERIOD. MEANWHILE RISING &nbspHEIGHTS APPEAR LIKELY ACROSS THE SOUTH CENTRAL U.S. AS THE &nbspSUBTROPICAL RIDGE SHIFTS WEST ACROSS THE GULF OF MEXICO. &nbsp &nbsp&nbsp &nbsp..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES &nbsp &nbspCONFIDENCE HAS ACTUALLY DECREASED A BIT FOR SPECIFICS OF &nbspCENTRAL-EASTERN U.S. EVOLUTION DAY 4 WED ONWARD. THE 00Z ECMWF &nbspHAS FURTHERED ITS TREND INTRODUCED IN ITS 12Z/23 RUN, FEATURING &nbspNORTHERN TIER U.S. FLOW THAT IS SUFFICIENTLY PROGRESSIVE TO RESULT &nbspIN SLOWER TIMING OF THE UPPER LOW ENERGY EJECTING FROM THE EXTREME &nbspSOUTHWEST TUE-WED. THE ECMWF SCENARIO ALOFT LEADS TO &nbspSLOWER/SOUTHWARD LOW PRESSURE FROM THE PLAINS EASTWARD AND THEN &nbspINTERACTION WITH NORTHERN STREAM ENERGY PRODUCES A VERY &nbspDEEP/WESTWARD SYSTEM JUST OFF THE EAST COAST BY EARLY DAY 7 SAT. &nbspTHE 00Z UKMET ADJUSTED TO THE SLOWER TIMING THROUGH THE DAY THU. &nbspTHE ENSEMBLE MEANS, AND THE ECMWF MEAN IN PARTICULAR, HAVE &nbspPROVIDED THE MOST STABLE FORECAST THUS FAR IN MAINTAINING A PHASED &nbspUPPER TROUGH WHICH SUPPORTS A SURFACE LOW TRACK FROM THE &nbspSOUTH-CENTRAL PLAINS THROUGH MID-MS VALLEY AND LOWER GREAT LAKES &nbspFOLLOWED BY SECONDARY DEVELOPMENT OFF THE EAST COAST. THE GFS/CMC &nbspHAVE LOOSELY FOLLOWED THIS IDEA IN PRINCIPLE ALBEIT WITH SOME &nbspSIGNIFICANT DIFFERENCES IN TIMING/STRENGTH. SOME GFS RUNS MAY BE &nbspA LITTLE FAR NORTHWARD WITH THE LOW PRESSURE TRACK OVER THE &nbspNORTHEAST QUADRANT OF THE U.S. IN LIGHT OF THE RIDGE RETROGRADING &nbspACROSS SOUTHERN GREENLAND. THE 00Z GFS COMPARES BETTER TO &nbspCONSENSUS IN THIS REGARD THAN THE 06Z RUN. OVER THE PAST FEW DAYS &nbspECMWF RUNS HAVE BEEN VERY INCONSISTENT WITH EJECTION OF THE &nbspSOUTHWEST U.S. UPPER LOW/TROUGH AND THE MEAN PATTERN APPEARS TO &nbspFAVOR REASONABLE PROGRESSION. THUS FOR THE PURPOSES OF A SINGLE &nbspDETERMINISTIC FORECAST, PREFER TO MAINTAIN A SOLUTION CLOSE TO OR &nbspSLIGHTLY DEEPER THAN CONTINUITY BY WAY OF THE 06Z GEFS MEAN (MUCH &nbspBETTER DEFINED THAN THE 00Z RUN) AND 00Z ECMWF MEAN ALONG WITH &nbspSOME 00Z GFS/CMC INPUT. SOLUTIONS ARE CLOSE ENOUGH TO ALLOW FOR &nbspSOME ECMWF INCLUSION TUE INTO WED. &nbsp &nbspALONG THE WEST COAST THE 06Z GFS COMPARES LESS FAVORABLY TO MOST &nbspGUIDANCE WITH A FARTHER SOUTH/SOUTHEAST TRACK OF UPPER &nbspENERGY/SURFACE LOW PRESSURE NEARING THE PACIFIC NORTHWEST BY LATE &nbspDAY 4 WED. SO AS WITH THE FORECAST BLEND FARTHER EAST, PREFERENCE &nbspGOES TO THE 00Z RUN FOR THE GFS COMPONENT. THEN THERE IS &nbspREASONABLE AGREEMENT THAT AMPLIFYING FLOW ALOFT WILL SUPPORT A &nbspFAIRLY DEEP UPPER LOW SETTLING JUST OFF THE PACIFIC NORTHWEST &nbspCOAST BY DAY 6 FRI. TOWARD THE LATTER HALF OF THE PERIOD &nbspMODEL/ENSEMBLE SOLUTIONS BEGIN TO DIVERGE WITH RESPECT TO HOW MUCH &nbspUPSTREAM ENERGY CUTS THROUGH THE EAST-CENTRAL PACIFIC RIDGE AND &nbspTHE EFFECT OF THIS ENERGY ON WEST COAST TROUGH PROGRESSION BY &nbspEARLY NEXT WEEKEND. GFS RUNS ARE STILL A BIT ON THE &nbspSTRONG/PROGRESSIVE SIDE WITH THE UPSTREAM ENERGY-- EVENTUALLY &nbspLEADING TO A FASTER WESTERN TROUGH ALOFT-- BUT THEY ARE MUCH &nbspIMPROVED OVER THEIR RUNS FROM 24 HOURS AGO WHICH HAD NEARLY FLAT &nbspFLOW OVER THE EAST PACIFIC/WESTERN U.S. BY FRI. EVEN IF THE GFS &nbspIS SOMEWHAT EXTREME, THERE ARE GROWING SIGNALS FROM MODEL/ENSEMBLE &nbspGUIDANCE AS A WHOLE THAT ENOUGH ENERGY MAY COME THROUGH THE RIDGE &nbspTO RESULT IN MORE WEST COAST TROUGH PROGRESSION BY NEXT SAT THAN &nbspADVERTISED BY THE 00Z CMC MEAN. OVERALL PREFERRED A 00Z MODEL &nbspCONSENSUS TRANSITIONING TOWARD THE 06Z GEFS/00Z ECMWF MEANS. LATE &nbspIN THE PERIOD MINORITY INPUT FROM THE 00Z GFS/CMC USED FOR THE &nbspEASTERN SYSTEM OFFSET FAST/SLOW TRAITS RESPECTIVELY FOR THE WEST &nbspCOAST TROUGH TO KEEP THE FORECAST ANCHORED NEAR THE MEANS. &nbsp &nbsp&nbsp &nbsp..WEATHER HIGHLIGHTS/THREATS &nbsp &nbspMID-LATE WEEK UPPER TROUGH SETTLING ALONG THE WEST COAST AND &nbspPOSSIBLE EMBEDDED CLOSED LOW NEARING THE PACIFIC NORTHWEST WILL &nbspBRING INCREASING POTENTIAL FOR AREAS OF HEAVY RAIN/HIGHER &nbspELEVATION SNOW OVER PORTIONS OF THE WEST. EXPECT HIGHEST 5-DAY &nbspTOTALS TO EXTEND FROM THE COASTAL/CASCADE AREAS OF THE PACIFIC &nbspNORTHWEST DOWN INTO NORTHERN CALIFORNIA AND THE SIERRA NEVADA &nbspRANGE. SOME ENHANCED PRECIP IS ALSO POSSIBLE ALONG THE REST OF &nbspTHE CA COAST AND FARTHER INLAND INTO THE ROCKIES. ARRIVAL OF &nbspUPPER TROUGHING WILL LEAD TO A DECREASE IN SNOW LEVELS. AHEAD OF &nbspTHIS LARGE SCALE TROUGH, A SHORTWAVE/EMBEDDED CLOSED LOW TRACKING &nbspTHROUGH THE EXTREME SOUTHWEST EARLY IN THE PERIOD WILL BE &nbspACCOMPANIED BY SOME RAIN AND HIGHER ELEVATION SNOW. EXPECT BELOW &nbspNORMAL TEMPS TO PERSIST OVER MUCH OF THE WEST INTO THE NORTHERN &nbspHIGH PLAINS THROUGH THE PERIOD, WITH SOME LOCATIONS SEEING ONE OR &nbspMORE DAYS OF MINUS 10-20F ANOMALIES. &nbsp &nbspWIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN &nbspPOSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY INTO THE TN &nbspVALLEY, AND PERHAPS SOMEWHAT NORTH OR SOUTH OF THIS REGION, FROM &nbspLATE TUE THROUGH THU NIGHT AS MOIST LOW-LEVEL SOUTHERLY FLOW &nbspWITHIN A CYCLONE'S WARM CONVEYOR BELT SPREADS AMPLE MOISTURE &nbspNORTHWARD OVER A RETURNING WARM FRONT. FARTHER NORTH LOW PRESSURE &nbspTRACKING ACROSS THE PLAINS/MIDWEST/GREAT LAKES WED-THU WOULD BRING &nbspCHANCES OF SNOWFALL TO LOCATIONS NORTH OF THE LOW TRACK. &nbspCONFIDENCE IS LOW IN THE SPECIFICS AT THIS POINT GIVEN SIGNIFICANT &nbspDIFFERENCES IN SOME GUIDANCE, BUT POTENTIAL EXISTS FOR HEAVY RAIN &nbspTO EXTEND INTO PORTIONS OF THE MID-ATLANTIC OR NORTHEAST THU-FRI &nbspWITH SNOW POSSIBLE FOR NEW ENGLAND. BROAD POSITIVE HEIGHT &nbspANOMALIES/UPSTREAM SURFACE LOW PRESSURE WILL KEEP TEMPERATURES &nbspABOVE AVERAGE FOR MUCH OF THE CENTRAL/EASTERN U.S. TUE-WED WITH &nbspMIN TEMPS IN PARTICULAR LIKELY TO BE MUCH ABOVE NORMAL (POSSIBLY &nbspEXCEEDING RECORD WARM VALUES AT SOME LOCATIONS). DEPARTURE OF LOW &nbspPRESSURE INTO THE WESTERN ATLANTIC SHOULD BRING TEMPS DOWN CLOSER &nbspTO NORMAL BY FRI-SAT. &nbsp &nbspRAUSCH/RYAN &nbsp &nbsp &nbsp